based on the assumption that you have a cradle, and are not breaking any laws by holding your handset whilst driving...
The easiest method I've found is as described above.
Hit the 'one button' to call from the nokia handset.
Leave it for (literally) a second or two - after it has started to call, but before it actually rings
Hang up (on the nokia handset)
Then, press the HFT voice button on the steering wheel
Watch in amazement as the HFT system tells the Nokia to redial the last number it called - ie the one from the email.
There is another way, which involves making the call from the nokia handset, then somehow transferring the call into the HFT system.
But I haven't got round to reading that part of the manual yet